The convergence toward a packet based wireless network started to take place with the Release 5 of the WCDMA 3GPP standard, with the introduction of adaptive modulation and coding on the High Speed Physical Downlink Shared Channel (HS-PDSCH) to exploit the good channel conditions of the user equipment in order to achieve high instantaneous data rates and thus maximise the system performance. The use of multiple antennas at the base station (node-B in WCDMA terminology) and multiple antennas at the user equipment (UE) allows even higher peak rates to be achieved. To exploit this advantage, the Multiple-Input-Multiple-Output (MIMO) HS-PDSCH mode has recently been introduced in Release 7 of WCDMA 3GPP standard [3GPP TS 25.214, “Technical Specification Group Radio Access Network; Physical layer procedures (FDD)”, Mars 2008, Section 9].
As shown in FIG. 1a, under Release 7 a node-B operating in MIMO mode can transmit up to two HS-PDSCH data streams, each having different antenna precoding weights, which are used to minimise the inter-stream interference at the receiver.
FIG. 1a shows a schematic block diagram of a part of a node-B transmitter 2 operable in a MIMO mode. In operation, primary transport blocks are processed through a primary transport processing module 41 then multiplied by the node-B's scrambling code and the UE's spreading codes at a primary code-division multiplier 61 to generate a primary HS-PDSCH stream. A first instance of the primary HS-PDSCH stream is then multiplied by a first primary weighting factor w1 at a first primary weighting multiplier 81 for transmission to the UE in question via a first antenna 141, and a second instance of the primary HS-PDSCH stream is multiplied by a second primary weighting factor w2 at a second primary weighting multiplier 82 for transmission to the same UE via a second antenna 142. Thus the primary HS-PDSCH stream is transmitted to the UE via both antennas 141 and 142 but with different weightings applied.
If the node-B scheduler has selected to transmit two HS-PDSCH streams to the UE simultaneously, then in addition secondary transport blocks are processed through a secondary transport processing module 42 then multiplied by the node-B's scrambling code and the same UE's spreading code at a secondary code-division multiplier 61 to generate a secondary HS-PDSCH stream. A first instance of the secondary HS-PDSCH stream is then multiplied by a first secondary weighting factor w3 at a first secondary weighting multiplier 83 for transmission to the UE via the first antenna 141, and a second instance of the primary HS-PDSCH stream is multiplied by a second secondary weighting factor w4 at a second secondary weighting multiplier 82 for transmission to the same UE via the second antenna 142. Thus the secondary HS-PDSCH stream is also transmitted to the UE via both antennas 141 and 142 with different weightings applied. The first instances of the primary and secondary HS-PDSCH streams are summed at a first initial adder 101, and the second instances of the primary and secondary HS-PDSCH streams are summed at a second initial adder 102. Also, the output of the first initial adder 101 is summed with a first Common Pilot Channel CPICH1 at a first additional adder 121, and the output of the second initial adder 102 is summed with a second Common Pilot Channel CPICH2 at a second additional adder 122.
As shown schematically in FIG. 1b, each transmit antenna 141 and 142 transmits to each (both) receive antennas 161 and 162 at the UE's receiver. It is then the receiver's job to extract the two individual primary and secondary data streams from the different combinations of instances received at the two respective receive antennas 161 and 162.
Thus the node-B scheduler can select to transmit either one or two transport blocks to a UE in a given transport time interval, and thus is able to transmit up to two corresponding HS-PDSCH streams derived from such blocks.
Similar arrangements may be applied for transmission of other primary and secondary streams to other UEs, by multiplying by different spreading codes as will be familiar to a person skilled in the art. This is indicated in FIG. 1 by the dotted repetition of the diagram backwards into the page.
Note also that the arrangement can be generalised to any number n=1 . . . N of streams and any number m=1 . . . M of antennas, with each stream being transmitted from all M antennas, and each combination of stream n with antenna m being weighted by a respective weighting factor wn,m. In a general case, the maximum number of streams transmitted need not equal the number of transmit and/or receive antennas.
The introduction of MIMO mode in the WCDMA 3GPP system requires the UE to have the capability of estimating the Signal-to-Interference Ratio (SIR) between the primary and secondary streams, in order to be able to demodulate the received signal and generate the composite PCI and CQI. The Precoding Control Indication (PCI) is an indication of the UE's preferred weights. The Channel Quality Indicator (CQI) is a metric calculated by the UE based on the estimated SIR and fed back to the node-B on the uplink. The node-B can then use the reported CQI to adjust subsequent transmissions to the UE in order to improve performance as described for example in the 3GPP specifications.
Note: noise plus interference ({circumflex over (P)}iN) includes both the interference between streams and between CDMA spreading codes (even codes of the same user). For example, in the expression {circumflex over (P)}iN={circumflex over (P)}iv+{circumflex over (γ)}ÎiS,CPICH, {circumflex over (P)}iv is the noise plus interference between codes (by what is strictly a misnomer this is sometimes referred to just as noise), where {circumflex over (γ)}ÎiS,CPICH: is the interference between streams for the same code. This terminology will be used in the description below.
The MIMO mode is thus applied to the High Speed Physical Downlink Shared Channel (HS-PDSCH) and uses precoding weights to improve performance. However, due to the precoding of the transmitted signal and in the absence of dedicated pilots on the HS-PDSCH new techniques need to be applied to estimate the SIR.
In general, when precoding weights are used, either (a) dedicated pilots are transmitted to the UE on the same physical channel as the data stream or (b) the precoding weights are signalled to the UE on a different physical channel.
The dedicated pilots are used in the closed loop transmit diversity mode 1 for the Downlink Physical Channel (DPCH) of the WCDMA 3GPP standard [3GPP TS 25.211, “Technical Specification Group Radio Access Network; Physical Channels and Mapping of Transport Channels onto Physical Channels (FDD)”, December 2005, Section 5.3], and were proposed as well in [Brian Banister, “Adaptive Antenna Method and Apparatus,” U.S. Pat. No. 6,952,455, filed Oct. 4, 2005]. When being used, the dedicated pilots allow direct estimation of the effective channel experienced by the data stream, which is the composite of the precoding weights and the radio channel. The dedicated pilots also allow the estimation of the SIR at the output of the receiver front end (rake processor or equaliser), which captures any imperfection introduced in the receiver processing.
Alternatively, signalling the precoding weights on a different physical channel can save some of the transmit resources that the dedicated pilots consume. However, this also has the downside that the receiver has no direct access to the effective channel experienced by the data stream, which must be instead calculated using the signalled weights and the channel estimated from the Common Pilot Channel (CPICH). The estimation of the SIR in this case is currently done by a formula using the effective channel, the noise measured on the CPICH and the calculated equaliser (or rake) coefficients [R4-070180, Signal model for multi-stream Type 3 reference receiver, Ericsson, February 2007].
Calculating the SIR by this formula does not capture the imperfection of the receiver, and in the case of MIMO is unable to evaluate the inter-stream interference due to the channel estimation errors.